Properly handle empty stack traces in package:stack_trace.

pkg/stack_trace/test/trace_test.dart

Index: pkg/stack_trace/test/trace_test.dart
diff --git a/pkg/stack_trace/test/trace_test.dart b/pkg/stack_trace/test/trace_test.dart
index 5d34de5b9bdbf4d954285320a670ed391a8767cf..95753a0d240382730361045124c1aa3937cc1c27 100644
--- a/pkg/stack_trace/test/trace_test.dart
+++ b/pkg/stack_trace/test/trace_test.dart
@@ -102,7 +102,9 @@ void main() {
     });
 
     test('parses an empty string correctly', () {
-      expect(new Trace.parse('').frames, isEmpty);
+      var trace = new Trace.parse('');
+      expect(trace.frames, isEmpty);
+      expect(trace.toString(), equals(''));
     });
   });